Solid-State Light-Emitting Electrochemical Cells Based on ... · Solid-State Light-Emitting Electrochemical Cells 107 achieved by forming a heterostructure device, thus moving the

Solid-State Light-Emitting Electrochemical Cells

Based on Cationic Transition Metal Complexes

for White Light Generation

Hai-Ching Su, Ken-Tsung Wong, and Chung-Chih Wu

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

1.1 Features of Light-Emitting Electrochemical Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

1.2 Organization of This Chapter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

2 Review of Light-Emitting Electrochemical Cells Based on Cationic Transition

Metal Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

2.1 Increasing Device Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

2.2 Color Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

2.3 Lengthening Device Lifetime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

2.4 Shortening Turn-On Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

3 White Light-Emitting Electrochemical Cells Based

on Cationic Transition Metal Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

3.1 Photophysical and Electrochemical Properties of Novel

Blue-Green and Red-Emitting Cationic Iridium Complexes . . . . . . . . . . . . . . . . . . . . . . . . 127

3.2 Electroluminescent Properties of White Light-Emitting Electrochemical

Cells Based on Host–Guest Cationic Iridium Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

4 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134

Abstract Solid-state light-emitting electrochemical cells (LECs) based on cationic

transition metal complexes (CTMCs) exhibit several advantages over conventional

light-emitting diodes such as simple fabrication processes, low-voltage operation,

H.-C. Su (*)

Institute of Lighting and Energy Photonics, National Chiao Tung University, No. 301, Gaofa 3rd

Road, Guiren Township, Tainan County 711, Taiwan, ROC

e-mail: [email protected]

K.-T. Wong

Department of Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 106,

Taiwan, ROC

C.-C. Wu (*)

Department of Electrical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road,

Taipei 106, Taiwan, ROC

e-mail: [email protected]

V.W.W. Yam (ed.), WOLEDs and Organic Photovoltaics, Green Energy and Technology,

DOI 10.1007/978-3-642-14935-1_4, # Springer-Verlag Berlin Heidelberg 2010

105

and high power efficiency. Hence, white CTMC-based LECs may be competitive

for lighting applications. In this chapter, we review previous important works on

CTMC-based LECs, such as increasing device efficiency, color tuning, lengthening

device lifetime, and shortening turn-on time. Our demonstration of white CTMC-

based LECs by using the host–guest strategy is then described.

1 Introduction

1.1 Features of Light-Emitting Electrochemical Cells

White organic light-emitting diodes (OLEDs) based on polymers and small-molecule

materials have attracted much attention because of their potential applications in

flat-panel displays and solid-state lighting [1–5]. Compared with conventional

white OLEDs [6, 7], solid-state white light-emitting electrochemical cells (LECs)

[8–10] possess several promising advantages. LECs generally require only a single

emissive layer, which can be easily processed from solutions, and can conveniently

use air-stable electrodes. The emissive layer of LECs contains mobile ions, which

can drift toward electrodes under an applied bias. The spatially separated ions

induce doping (oxidation and reduction) of the emissive materials near the electro-

des, that is, p-type doping near the anode and n-type doping near the cathode

[8–10]. The doped regions induce ohmic contacts with the electrodes and conse-

quently facilitate the injection of both holes and electrons, which recombines at the

junction between p- and n-type regions. As a result, a single-layered LEC device

can be operated at very low voltages (close to Eg/e, where Eg is the energy gap of

the emissive material and e is elementary charge) with balanced carrier injection,

giving high power efficiencies. Furthermore, air-stable metals, for example, gold

and silver, can be used since carrier injection in LECs is relatively insensitive to

work functions of electrodes. Therefore, the power-efficient properties and easy

fabrication processes make LECs competitive in solid-state lighting technologies.

In the past few years, cationic transition metal complexes (CTMCs) have also

been increasingly studied for solid-state LECs because of the following several

advantages over conventional LECs or polymer LECs (a) ion-conducting material

is not needed since these metal complexes are intrinsically ionic and (b) higher

electroluminescent (EL) efficiencies could be achieved due to the phosphorescent

nature of the transition metal complexes. The development of solid-state LECs

based on CTMCs thus will be described and discussed in this chapter.

1.2 Organization of This Chapter

This chapter is organized as follows. Section 2 reviews advances of LECs based on

CTMCs, including topics of device efficiency (Sect. 2.1), colors (Sect. 2.2), device

106 H.-C. Su et al.

lifetimes (Sect. 2.3), and turn-on times (Sect. 2.4). Section 3 is devoted to white

LECs based on CTMCs. In Sect. 3.1, photophysical and electrochemical properties

of newly developed blue-green and red emitting cationic iridium complexes are

discussed. EL properties of white LECs based on host–guest cationic iridium

complexes are then discussed in Sect. 3.2. Finally, we give an outlook about

white LECs based on CTMCs.

2 Review of Light-Emitting Electrochemical Cells Based

on Cationic Transition Metal Complexes

The first solid-state LEC was demonstrated with a polymer blend sandwiched

between two electrodes [8]. The polymer blend was composed of an emissive

conjugated polymer, a lithium salt (lithium trifluoromethanesulfonate), and an

ion-conducting polymer (polyethylene oxide). The salt provides mobile ions and

the ion-conducting polymer prevents this blend film from phase separation that may

be induced by polarity discrepancy between the conjugated polymer and the salt.

More recently, CTMCs have also been used in LECs, which show several advan-

tages over conventional polymer LECs. In such devices, no ion-conducting material

is needed since these metal complexes are intrinsically ionic. They generally show

good thermal stability and charge-transport properties. Furthermore, high EL effi-

ciencies are expected due to the phosphorescent nature of these metal complexes.

2.1 Increasing Device Efficiency

The first solid-state LEC based on transition metal complexes was reported in 1996

[11], where a ruthenium poly-pyridyl complex was utilized as the emissive material.

Since then, many efforts have been made to improve performances of the LECs. In

1999, LECs based on low-molecular-weight ruthenium complexes were reported to

have external quantum efficiency (EQE), which is defined as photons/electrons, of

1% [12]. In 2000, LECs using [Ru(bpy)3](ClO4)2 (where bpy is 2,20-bipyridine) with

the gallium–indium eutectic electrode showed an EQE up to 1.8% [13]. Later,

[Ru(bpy)3](PF6)2 blended with poly(methyl methacrylate) (PMMA) was reported

to improve the film quality and increase the EQE to 3% [14]. In 2002, a single-

crystal LEC was made by repeatedly filling nearly saturated solution of [Ru(bpy)3]

(ClO4)2 between two ITO (indium tin oxide) slides, followed by evaporating the

solvent [15]. Such devices possessed low turn-on voltages and exhibited an EQE of

3.4%. Further improvement of performances was achieved by reducing self-quench-

ing of excited states in [Ru(bpy)3]2+ with adding alkyl substituents on the bpy

ligands [16], raising the EQE to 4.8% under the DC bias and to 5.5% under the

pulsed driving. Yet a further improvement of the [Ru(bpy)3](ClO4)2 device was

Solid-State Light-Emitting Electrochemical Cells 107

achieved by forming a heterostructure device, thus moving the emission zone away

from the electrode and giving an efficiency of 6.4% [17].

More efficient cationic iridium complexes have also been used in LECs. In

2004, LECs based on the yellow-emitting (560 nm) cationic iridium complex

[Ir(ppy)2(dtb-bpy)]PF6, where ppy is phenylpyridine and dtb-bpy is 4,40-di-tert-butyl-2,20-bipyridine, were reported, exhibiting efficiencies of 5% and 10 lm W�1

[18, 19]. On the one hand, replacing the ppy ligands by F-mppy ones, where

F-mppy is 2-(40-fluorophenyl)-5-methylpyridine, led to green emission (531 nm)

and an EL efficiency of 1.8% [19, 20]. On the other hand, employing the dF(CF3)ppy

ligands, where dF(CF3)ppy is 2-(2,4-difluorophenyl)-5-trifluoromethylpyridine,

increased the energy gap of the cationic iridium complexes and shifted the EL

emission to blue-green (500 nm, with an EQE of 0.75%) [21]. Another series of

cationic phenylpyrazole-based iridium complexes were also recently reported to

give blue (492 nm), green (542 nm), and red (635 nm) emission [22]. Blue, green,

and red devicesmade of these complexes on poly(3,4-ethylenedioxythiophene):poly

(styrene sulfonate) (PEDOT:PSS) coated ITO showed EQEs of 4.7%, 6.9%, and

7.4%, respectively.

Most LEC devices described above were made in a single-layered neat-film

structure. In a neat film of an emissive material, interactions between closely

packed molecules usually lead to quenching of excited states, detrimental to

EQEs of devices. Thus, in addition to tuning of emission colors, the capability of

the ligands to provide steric hindrance for suppressing self-quenching must also be

carefully considered in designing ligands for emissive cationic metal complexes. In

our previous studies of oligofluorenes, we had found that the introduction of aryl

substitutions onto the tetrahedral C9 of oligofluorenes can provide effective hin-

drance to suppress interchromophore packing and self-quenching yet without

perturbing energy gaps of the molecules [23–27], making the photoluminescent

quantum yields (PLQYs) of oligo(9,9-diarylfluorene)s in neat films rather close

to those in dilute solutions. Thus, we introduced 4,5-diaza-9,90-spirobifluorene(SB) [28] as a steric and bulky auxiliary ligand of cationic iridium complexes,

[Ir(ppy)2(SB)]PF6 (1) and [Ir(dFppy)2(SB)]PF6 (2) (Fig. 1), and investigate the

influence of the SB ligand on reducing the self-quenching [29].

Spin-coated neat films of 1 and 2 exhibit PL spectra similar to those observed

for their acetonitrile (MeCN) solutions (Fig. 2). However, spin-coated neat films of

1 and 2 show higher PLQYs (0.316 for 1, 0.310 for 2) and longer excited-state

lifetimes (0.60 ms for 1, 0.59 ms for 2) than their MeCN solutions (Table 1). Since

MeCN is a strongly polar solvent and the emitters are ionic, the photophysical

properties of complexes 1 and 2 perhaps are significantly perturbed by strong

solute–solvent interaction, rendering the observed PLQYs and lifetimes of 1 and 2 in

MeCN less intrinsic. This indeed can be verified by measuring photophysical

properties of 1 and 2 in a less polar solvent (yet still with enough solubility

for spectroscopic measurements), such as dichloromethane (DCM). In DCM

(5 � 10�5 M), both 1 and 2 exhibit longer excited-state lifetimes (0.79 ms for 1,0.42 ms for 2) and higher PLQYs (0.467 for 1, 0.364 for 2) than in MeCN (Table 1).

108 H.-C. Su et al.

Thus, to better characterize the intrinsic photophysical properties of complexes

1 and 2 at the room temperature, complexes 1 and 2were dispersed (with 1.5 mol%)

in a large-gap thin-film host of m-bis(N-carbazolyl)benzene (mCP), which is rather

nonpolar and has a large triplet energy. The measured PLQYs and excited-state

lifetimes of the dispersed mCP films are (0.667, 0.81 ms) for 1 and (0.421, 0.74 ms)for 2 (Table 1). Thus 1 and 2 dilutely dispersed in mCP exhibit higher PLQYs and

longer excited-state lifetimes than those in neat films. Shorter lifetimes in neat films

indicate that interaction between closely packed molecules provides additional

deactivation pathways, shortening excited-state lifetimes, and lowering the

PLQYs. However, PLQYs of complexes 1 and 2 in neat films still retain ~50%

and ~75% of those in mCP blend films. These are indeed rather high retaining

percentages for PLQYs in neat films when compared with other phosphorescent

molecules. For instance, [Ir(ppy)3] [1.5 mol% dispersed in 4,40-bis(carbazol-9-yl)

200 300 400 500 600 700 8000

1

2

3

4

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Abs

orba

nce

(a.u

.)

Wavelength (nm)

PL, Abs, SolutionPL, Abs, Neat filmEL

Inte

nsity

(a.

u.)

200 300 400 500 600 700 8000

1

2

3

4

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Abs

orba

nce

(a.u

.)

Wavelength (nm)

PL, Abs, SolutionPL, Abs, Neat filmEL

Inte

nsity

(a.

u.)

a b

Fig. 2 Absorption and PL spectra in acetonitrile solutions and in neat films of (a) complex 1 and

(b) complex 2. Also shown in (a) and (b) are EL spectra of complex 1 and complex 2, respectively.

Reproduced with permission from [29]. Copyright 2007, WILEY-VCH Verlag GmbH & Co.

KGaA, Weinheim

N N

Ir

N

N

FF

F

F

N N

Ir

N

N

+

PF6– PF6

–

+

1 2

Fig. 1 Molecular structures of complexes 1 and 2


biphenyl (CBP)] and bis[(4,6-difluorophenyl)pyridinato-N,C2](picolinato)iridium

(III) [FIrpic] (1.4 mol% dispersed in mCP) films had been reported to have very

high PLQYs of 97 � 2% and 99 � 1%, respectively [30], while PLQYs of

[Ir(ppy)3)] and [FIrpic] neat films are only ~3% and ~15%, respectively. On the

one hand, severe self-quenching in [Ir(ppy)3] and [FIrpic] implies that the ppy,

dFppy, and picolinic acid ligands cannot provide enough hindrance against inter-

molecular interactions in neat films. On the other hand, the high retaining percen-

tages in neat-film PLQYs of complex 1 (with two ppy ligands and one SB ligand)

and complex 2 (with two dFppy ligands and one SB ligand) compared with those in

dispersed films indicate that SB ligands in these compounds provide effective steric

hindrance and greatly reduce self-quenching. It is noted that self-quenching in neat

films of [FIrpic] is not as severe as that in neat films of [Ir(ppy)3]. It is likely that the

fluoro substituents on the ppy ligands somewhat hinder the intermolecular interac-

tions [30]. A similar effect is also observed here for complex 2, which exhibits a

higher PLQY retaining percentage (74%) than complex 1.

In the operation of LEC devices, when a constant bias voltage is applied, a

delayed EL response that is associated with the time needed for counterions in the

LECs to redistribute under a bias is typically observed. For the cases of neat films of

complexes 1 and 2, the redistribution of the anions (PF6�) leads to the formation of

a region of Ir(IV)/Ir(III) complexes (p-type) near the anode and a region of Ir(III)/Ir

Table 1 Summary of physical properties of complexes 1 and 2

Complex lmax, PL (nm)a PLQY, lifetime (ms)b Eox1=2 (V)

cEred1=2 (V)

d DE1/2 (V)e

Solutionf Neat film Solution Film

1 605 593 0.226, 0.33f 0.316, 0.60g 1.35 �1.33 2.64

0.467, 0.79h 0.667, 0.81i

–, 4.31j 0.381, 0.72k

–, 3.27l

2 535 535 0.278, 0.39f 0.310, 0.59g 1.70 �1.26 2.92

0.364, 0.42h 0.421, 0.74i

–, 4.49j 0.329, 0.55k

–, 4.29l

aPL peak wavelengthbPhotoluminescence quantum yields and the excited-state lifetimescOxidation potentialdReduction potentialeThe electrochemical gap DE1/2 is the difference between E

ox1=2 and E

red1=2 corrected by potentials of

the ferrocenium/ferrocene redox couplefMeasured in acetonitrile (5 � 10�5 M) at room temperaturegNeat filmshMeasured in dichloromethane (5 � 10�5 M) at room temperaturei1 and 2 were dispersed (1.5 mol%) in mCP filmsjMeasured in acetonitrile (5 � 10�5 M) at 77 KkFilms with 0.75 mol [BMIM][PF6] per mole of 1 and 2lMeasured in dichloromethane (5 � 10�5 M) at 77 K


KGaA, Weinheim

110 H.-C. Su et al.

(II) complexes (n-type) near the cathode [31]. With the formation of p- and

n-regions near the electrodes, carrier injection is enhanced, leading to a gradually

increasing device current and emission intensity. Devices based on neat films of

complexes 1 and 2 (with the structure of glass substrate/ITO/complex 1 or 2 (100 nm)/

Ag) exhibited very long response times. Very slow device response (e.g., tens of

hours to reach the maximum brightness) had also been observed before for other

LEC materials (e.g., [Ru(bpy)3](PF6)2 derivatives with esterified ligands) [12].

Plausibly, bulky side groups on molecules impede the migration of ions. Thus to

accelerate formation of the p- and n-doped regions in the emissive layer, 0.75 mol

ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6] per

mole of complex 1 or 2 was added to provide additional anions (PF6�) [32].

It had been reported that in polymer LECs, incorporation of polar salts into

conjugated polymer films might induce aggregates or phase separation due to

discrepancy in polarity [33–35]. Thus, to study the effect of the [BMIM][PF6]

addition on thin-film morphologies of complexes 1 and 2, atomic force microscopy

(AFM) of thin films was performed. As shown in Fig. 3, the AFM micrographs for

films of complexes 1 and 2 with and without [BMIM][PF6] coated on ITO glass

substrates show no significant differences and all give similar root-mean-square

(RMS) roughness of ~1 nm. At this concentration of [BMIM][PF6] in complex 1 or 2

(0.75:1, molar ratio), no particular features of aggregation or phase separation were

a b

c d

1000

10

800

600

400

200

0200

400600

8001000

00

5

10

15

nm

nm

nm

nm1000

10

800600

400

200

0200

400600

8001000

00

5

10

15

nm

nm

nm

nm

1000

10

800600

400200

0 0200

400600

8001000

0

5

10

15

nm

nm

nm

nm1000

10

800600

400200

00

200400

600800

1000

0

5

10

nm

nm

nm

nm

Fig. 3 3D AFM micrographs of (a) neat film of complex 1, (b) blend film of [BMIM][PF6] and

complex 1 (0.75:1, molar ratio), (c) neat film of complex 2, and (d) blend film of [BMIM][PF6] and

complex 2 (0.75:1, molar ratio). Reproduced with permission from [29]. Copyright 2007, WILEY-

VCH Verlag GmbH & Co. KGaA, Weinheim


observed, and uniform spin-coated thin films could be routinely obtained. Such

characteristics may be associated with the ionic nature of complexes 1 and 2, which

may make them more compatible with the added salts. Further, to examine the

effects of the [BMIM][PF6] addition on photophysical properties of complexes

1 and 2, PLQYs and excited-state lifetimes of blend films were also measured and

are shown in Table 1. In general, films of both 1 and 2 containing [BMIM][PF6]

(1:0.75, molar ratio) exhibit PLQYs and excited-state lifetimes comparable to those

of neat films, indicating that the addition of [BMIM][PF6] does not induce particu-

lar quenching of emission. Stable operation was also achieved in devices using such

a formulation. In the following, device characteristics based on the structure of

[glass substrate/ITO/1:[BMIM][PF6] or 2:[BMIM][PF6] (100 nm)/Ag] are dis-

cussed and are summarized in Table 2.

A distinct characteristic of LECs is that they can be operated under a bias voltage

close to Eg/e. As shown in Table 1, the electrochemical gaps (DE1/2), which were

derived from the difference between Eox1=2 and Ered

1=2 corrected with potentials of the

ferrocenium/ferrocene redox couple, for complexes 1 and 2 are 2.64 eV and

2.92 eV, respectively. The devices based on complexes 1 and 2 were thus first

tested under the biases of 2.6 V and 2.9 V, respectively, although the energy gaps in

films are usually smaller than those in solutions because of the environmental

polarization. EL spectra of the devices based on complexes 1 and 2 (added with

[BMIM][PF6] are shown in Figs. 2a and 2b, respectively, for comparison with their

PL spectra. EL spectra are basically similar to PL spectra, indicating similar

emission mechanisms. Commission internationale de l’Eclairage (CIE) coordinates

for the EL spectra of complexes 1 and 2 are (0.51, 0.48) and (0.35, 0.57), respec-

tively. Time-dependent brightnesses and current densities of the devices operated

under bias conditions described above are shown in Figs. 4a and 4b for complexes 1

and 2, respectively. Both devices exhibited similar electrical characteristics. On the

one hand, the currents of both devices first increased with time after the bias was

applied and then stayed at a constant level after 350–400 min. On the other hand,

Table 2 Summary of the LEC device characteristics based on complexes 1 and 2

Complex Bias (V) lmax, EL (nm)a tmax (min)b Lmax cd m�2 �ext, max, �p, max

(%, lm W�1)dLifetime (h)e

1 2.6 580 170 330 6.2, 19.0 26

2.5 150 100 7.1, 22.6 54

2 2.9 535 85 145 6.6, 23.6 6.7

2.8 90 52 7.1, 26.2 12aEL peak wavelengthbTime required to reach the maximal brightnesscMaximal brightness achieved at a constant bias voltagedMaximal external quantum efficiency and maximal power efficiency achieved at a constant bias

voltageeThe time for the brightness of the device to decay from the maximum to half of the maximum

under a constant bias voltage


KGaA, Weinheim

112 H.-C. Su et al.

the brightness first increased with the current and reached the maximum of

330 cd m�2 for complex 1 at ~170 min and of 145 cd m�2 for complex 2 at

~85 min. The brightness then decreased with time even though the device current

stayed rather constant. The decrease in brightness was irreversible, that is, the

maximum brightness obtained in the first measurement could not be fully recovered

in the followed measurements even under the same driving conditions. It is ratio-

nally associated with the degradation of the emissive material during the LEC

operation, which was commonly seen in LEC devices [36].

Time-dependent EQEs and corresponding power efficiencies of the complex 1

device under the 2.6-V bias and the complex 2 device under the 2.9-V bias are

shown in Figs. 5a and 5b, respectively. Both devices exhibited similar time evolu-

tion in EQE. When a forward bias was just applied, the EQE was rather low due to

unbalanced carrier injection. During the formation of the p- and n-type regions near

electrodes, the balance of the carrier injection was improved and the EQE of the

device thus increased rapidly. The peak EQE and the peak power efficiency are

0

2

4

6

8

0

4

8

12

16

20

24

2.6 V2.5 V

Ext

erna

l Qua

ntum

Effi

cien

cy (

%)

Time (min)

Pow

er E

ffici

ency

(lm

/W)

0 100 200 300 400 500 600 0 100 200 300 400 500 6000

2

4

6

8

0

5

10

15

20

252.9 V2.8 V

Ext

erna

l Qua

ntum

Effi

cien

cy (

%)

Time (min)

Pow

er E

ffici

ency

(lm

/W)

ba

Fig. 5 The time-dependent EQE and the corresponding power efficiency of the single-layered

LEC device for (a) complex 1 driven at 2.6 or 2.5 V and (b) complex 2 driven at 2.9 or 2.8 V.


KGaA, Weinheim

0

50

100

150

200

250

300

350

0.0

0.5

1.0

1.5

2.0

2.5

2.5 V2.6 V

Brig

htne

ss (

cd/m

2 )

Brig

htne

ss (

cd/m

2 )

Time (min)

Cur

rent

Den

sity

(m

A/c

m2 )

Cur

rent

Den

sity

(m

A/c

m2 )

0 100 200 300 400 500 600 0 100 200 300 400 500 6000

25

50

75

100

125

150

0.00

0.25

0.50

0.75

1.00

1.25

2.8 V2.9 V

Time (min)

ba

Fig. 4 The time-dependent brightness and current density of the single-layered LEC device for

(a) complex 1 driven at 2.6 or 2.5 V and (b) complex 2 driven at 2.9 or 2.8 V. Reproduced with

permission from [29]. Copyright 2007, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim


(6.2%, 19.0 lm W�1) for the complex 1 device under the 2.6-V bias and (6.6%,

23.6 lm W�1) for the complex 2 device under the 2.9-V bias. One notices that the

peak efficiencies occurred before the currents reached their final maximal values.

Such a phenomenon may be associated with two factors. First, although with the

formation of the p- and n-type regions near electrodes, both contacts are becoming

more ohmic and the carrier injection at both electrodes are becoming more bal-

anced; however, the carrier recombination zone may consequently move toward

one of the electrodes because of discrepancy in electron and hole mobilities. The

recombination zone moving to the vicinity of an electrode may cause exciton

quenching such that the EQE of the device would decrease with time while the

current and the brightness are still increasing. Besides, degradation of the emissive

material under a high field would also contribute to the decrease in the EQE when

the recombination zone is still moving or when the recombination zone is fixed.

Both LEC devices driven under 0.1-V lower biases exhibit higher peak EQEs

and lower degradation rates. As shown in Figs. 5a and 5b, the peak EQE and the

peak power efficiency are (7.1%, 22.6 lm W�1) for the complex 1 device under the

2.5-V bias and (7.1%, 26.2 lm W�1) for the complex 2 device under the 2.8-V bias.

It is interesting to note that the peak EQEs of the single-layered devices (no

PEDOT:PSS is used) based on complexes 1 and 2 approximately approach the

upper limits that one would expect from the PLQYs of their neat films, when

considering an optical out-coupling efficiency of ~20% from a typical layered

light-emitting device structure. To our knowledge, these EQEs are among the

highest values reported for orange-red (or yellow) and green solid-state LECs

based on CTMCs. Such results indicate that CTMCs with superior steric hindrance

are essential and useful for achieving highly efficient solid-state LECs.

Since these newly developed cationic complexes (1 and 2) are intrinsically

efficient, to further reduce self-quenching and increase EL efficiency, one possible

approach is to spatially disperse the emitting complex (guest) into a matrix complex

(host), as previously reported for conventional OLEDs and solid-state LECs

[37–39]. The mixed host–guest films (~100 nm) for PL studies were spin-coated

onto quartz substrates using mixed solutions of various ratios. Since in LECs, a salt

[BMIM][PF6] of 19 wt% (where BMIM is 1-butyl-3-methylimidazolium) was also

added to provide additional mobile ions and to shorten the device response time

[29, 32], PL properties of the host–guest–salt three-component system were also

characterized.

Figure 6a shows the absorption spectrum of the guest and the PL spectra of the

host–guest two-component systems having various guest concentrations. With the

increase of the guest concentration, a gradual red shift from the host-like emission

to the guest-like emission is observed. As shown in Fig. 6b, the highest PLQY of

~37% (vs. ~31% of neat host and guest films) is obtained at the guest concentration

of 25 wt%, at which the emission is nearly completely from the guest. Accompany-

ing this enhanced PLQY is the longer excited-state lifetime (0.69 ms) when com-

pared with those of neat films (0.59 ms for the host and 0.60 ms for the guest),

indicating the effectiveness of the dispersion in suppressing quenching mechanisms

of guest molecules. Less complete transfer is observed at lower guest concentrations,

114 H.-C. Su et al.

0.00

0.04

0.08

0.12a

0.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

Wavelength (nm)

GuestAbsorption

GuestConcentration

0 wt.%5 wt.%15 wt.%25 wt.%50 wt.%100 wt.%

PL

Inte

nsity

(a.

u.)

30

35

40

45

50

b

Pho

tolu

min

esce

nce

Qua

ntum

Yie

ld (

%)

Guest Concentration (wt.%)

400 500 600 700 800

0 20 40 60 80 100

0 20 40 60 80 100

0.60

0.65

0.70

0.75

0.80

0.85

Exc

ited-

Sta

te L

ifetim

e (μ

s) Without BMIM+(PF6–)

With BMIM+(PF6–)

Without BMIM+(PF6–)

With BMIM+(PF6–)


c

Fig. 6 (a) The absorption spectrum of the neat guest film and PL spectra of host–guest films with

various guest concentrations (without [BMIM][PF6]. (b) Photoluminescence quantum yields and

(c) excited-state lifetimes as a function of the guest concentration for host–guest films without and

with [BMIM][PF6] (19 wt%). Reproduced with permission from [49]. Copyright 2006, American

Institute of Physics


which may be associated with the less extensive overlap between the host emission

and the weak guest absorption. A calculation of the F€orster radius for the host–guestenergy transfer gives a small value of <16 A [40] indicating inefficient energy

transfer at lower guest concentrations. With the addition of [BMIM][PF6] (19 wt%),

the trend in PL properties [spectra (not shown), PLQYs (Fig. 6b), and lifetimes

(Fig. 6c)] as a function of the guest concentration is similar to those of the two-

component system. Yet, an even higher PLQY of ~50% (about 1.6� enhancement

compared with PLQYs of neat host and guest films) and longer excited-state

lifetime of ~0.82 ms are observed around the guest concentration of 25 wt%. It

appears that [BMIM][PF6] not only provides additional mobile ions but is also

effective in suppressing interchromophore quenching.

Figure 7a shows the time-dependent brightness and current density under con-

stant biases of 2.5–2.7 V for the LEC using the mixture giving the highest PLQY

(i.e., with host, guest, and [BMIM][PF6] concentrations of 56 wt%, 25 wt%, and

19 wt%, respectively). After the bias was applied, on the one hand, the current first

increased and then stayed rather constant. On the other hand, the brightness first

increased with the current and reached the maxima of 10, 30, and 75 cd m�2 at <1 h

under biases of 2.5 V, 2.6 V, and 2.7 V, respectively. The brightness then dropped

with time with a rate significantly depending on the bias voltage (or current).

Corresponding time-dependent EQEs and power efficiencies of the same device

are shown in Fig. 7b. When a forward bias was just applied, the EQE was rather low

due to poor carrier injection. During the formation of the p- and n-type regions near

electrodes, the capability of carrier injection was improved and the EQE thus rose

rapidly. The peak EQE, current, and peak power efficiencies at 2.5 V, 2.6 V, and

2.7 V are (10.4%, 29.3 cd A�1, 36.8 lm W�1), (9.9%, 27.9 cd A�1, 33.7 lm W�1),

and (9.4%, 26.5 cd A�1, 30.8 lm W�1), respectively.

The maximum current density vs. voltage characteristics of LECs with various

guest concentrations are shown in Fig. 8a. The bias voltage required for same

0

20

40

60

80

0.0

0.1

0.2

0.3

0.4

0.5

2.5 V 2.6 V 2.7 V

Brig

htne

ss (

cd/m

2 )

Time (min)

Cur

rent

Den

sity

(m

A/c

m2 )

0 100 200 300 400 500 600 0 100 200 300 400 500 6000

2

4

6

8

10

12

0

5

10

15

20

25

30

35

2.5 V 2.6 V 2.7 V

Ext

erna

l Qua

ntum

Effi

cien

cy (

%)

Time (min)

Pow

er E

ffici

ency

(lm

/W)

a b

Fig. 7 (a) Brightness (solid symbols) and current density (open symbols) and (b) external

quantum efficiency (solid symbols) and power efficiency (open symbols) as a function of time

under a constant bias voltage of 2.5–2.7 V for the host–guest LEC with host, guest and [BMIM]

[PF6] concentrations of 56 wt%, 25 wt%, and 19 wt%, respectively. Reproduced with permission

from [49]. Copyright 2006, American Institute of Physics

116 H.-C. Su et al.

0.0

0.5

1.0

1.5

2.0

2.5–3.1 eV

HostGuest

–3.0 eV

–6.0 eV–5.7 eV

Guest (wt.%)

0

16

25

35

81

Max

imum

Cur

rent

Den

sity

(m

A/c

m2 )

Voltage (V)

0.0

0.2

0.4

0.6

0.8

1.0 GuestConcentration

0 wt.%7 wt.%

25 wt.%81 wt.%

EL

Inte

nsity

(a.

u.)

Wavelength (nm)

2.5 2.6 2.7 2.8 2.9

400 500 600 700 800

0 20 40 60 80

7

8

9

10

11

16

20

24

28

32

36

Quantum EfficiencyPea

k E

xter

nal

Qua

ntum

Effi

cien

cy (

%)


Power Efficiency

Pea

k P

ower

Effi

cien

cy (

lm/W

)

a

b

c

Fig. 8 (a) Maximum current density vs. voltage characteristics for LECs with various guest

concentrations. (b) EL spectra (at 2.8 V) for LECs with various guest concentrations. (c) Peak

external quantum efficiencies and peak power efficiencies (at current densities <0.1 mA cm�2) of

host–guest LECs as a function of the guest concentration. Inset of (a): the energy level diagram of

the host and guest molecules. Reproduced with permission from [49]. Copyright 2006, American

Institute of Physics


current drops as the guest concentration is raised. This may be understood by the

energy level diagram obtained from cyclic voltammetry [29] (inset of Fig. 8a). Such

an energy level alignment favors carrier injection/transport (at least for holes)

through the smaller-gap guest and direct carrier recombination/exciton formation

on the guest (rather than host–guest energy transfer) if the guest concentration is

high enough. EL spectra of host–guest LECs with various guest concentrations

(Fig. 8b) indeed support the mechanism of direct exciton formation on guest

molecules, which would greatly reduce host emission due to incomplete energy

transfer. Compared with PL spectra (e.g., Fig. 6), EL spectra are much less

dependent on the guest concentration. Even with a guest concentration as low as

7 wt%, the EL spectrum is almost the same as that of the pure guest device [i.e., the

LEC with 81 wt% of guest and 19 wt% of [BMIM][PF6], indicating predominant

guest emission.

Figure 8c shows peak EQEs and peak power efficiencies for host–guest LECs as

a function of the guest concentration (at current densities <0.1 mA cm�2). The

peak EQEs and power efficiencies roughly follow the trend of the PLQYs. The

highest peak EQE and power efficiency of 10.4% and 36.8 lm W�1 are achieved at

the guest concentration of 25 wt%, coincident with the concentration giving the

highest PL efficiency. Such an EQE represents a 1.5� enhancement compared with

those of pure host and guest devices. It is also worth noting that such a peak EQE

approximately approaches the upper limit that one would expect from the PLQY of

the host–guest emissive layer (~50%), when considering an optical out-coupling

efficiency of ~20% from a typical layered light-emitting device structure. Such

EQEs and power efficiencies also are among the highest reported for solid-state

LECs based on CTMCs. Such results indicate that the host–guest system is essential

and useful for achieving highly efficient solid-state LECs.

2.2 Color Tuning

Early developments of CTMCs for LECs were focused on ruthenium-based com-

plexes, which typically emit in the red and orange–red part of the spectrum due to

low metal-to-ligand charge transfer (MLCT) energies. Ester-substituted ligands

have been found to red-shift the emission of [Ru(bpy)3](PF6)2 [12, 13]. Ng et al.

[41] published a series of polyimides containing a Ru complex on the main chain.

The device showed deep-red EL centered at about 650 nm and near infrared (IR)

emission centered near 750–800 nm. However, the efficiency of such device is

rather low (0.1%). Bolink et al. [42] also have achieved a deep red emission, using

bis-chelated Ru complex [Ru(tpy)(tpy-CO2Et)](PF6)2, where tpy is terpyridine.

The ester substituents were found to significantly improve the device efficiency

over the unsubstituted terpyridyl complex, but the device efficiency is still very low.

Low efficiency is a general phenomenon of terpyridyl Ru complexes since the

luminescent 3MLCT state is quenched by low-lying 3MC levels due to poor bite

angles of the tpy ligands [43].

118 H.-C. Su et al.

Iridium was shown to have promise for color tuning over ruthenium complexes

because of increased ligand-field splitting energies. The pioneering work of Ir-

based complex ([Ir(ppy)2(dtb-bpy)]PF6 for LECs was reported by Slinker et al.

[18]. Subsequent color tuning of CTMCs has centered on appropriate selection of

ligand or transition metal core. Complexes based on fluorine substituents on the

phenylpyridine ligands were reported to cause a blue shift of the emission spectrum

relative to unfluorinated complexes. For example, ([Ir(ppy)2(dtb-bpy)]PF6 and [Ir

(dF(CF3)ppy)2(dtb-bpy)]PF6 showed EL centered at 542 nm and 500 nm, respec-

tively [20, 21]. The increasing of bandgap of complexes results from the fact that

metal-centered highest occupied molecular orbital (HOMO) level is highly stabi-

lized by introduction of the fluorine atoms [21]. We also studied the electrochemi-

cal characteristics of complexes 1 and 2 via cyclic voltammetry. As shown in Fig. 9,

complex 2 exhibits a reversible oxidation peak at 1.70 V vs. Ag/AgCl, which is

significantly higher than that of complex 1 (1.35 V). The HOMO level of ([Ir

(ppy)2(dtb-bpy)]PF6 had been reported to be a mixture of the d-orbitals of iridium

and the p-orbitals of the phenyl ring of the ppy ligand [21]. Since 1 and 2 are also

cationic ppy-based Ir(III) complexes, their HOMO distributions should be similar

to that of ([Ir(ppy)2(dtb-bpy)]PF6. The higher oxidation potential of complex

2 compared with that of complex 1 indicates that the two electron-withdrawing

fluoro substituents on the phenyl ring of the ppy ligand possibly reduce the electron

density on the Ir metal center and consequently stabilize the HOMO level. Accord-

ing to the previous studies, the lowest unoccupied molecular orbital (LUMO) of

([Ir(ppy)2(dtb-bpy)]PF6 is located mainly on the dtb-bpy ligand [21], and therefore,

for complexes 1 and 2, reduction is expected to take place on the auxiliary SB

ligand. Similar reduction potentials for complex 1 (–1.33 V) and complex

2 (–1.26 V) confirm that this common ligand is responsible for reduction of both

compounds. Still, the reduction potential of complex 2 is slightly less negative than

2.0 1.5 1.0 0.5 0.0 – 0.5 –1.0 –1.5

– 0.01

0.00

0.01 Complex 1Complex 2

Cur

rent

(m

A)

Potential (V vs. Ag / AgCl)

Fig. 9 Cyclic voltammograms of complexes 1 and 2. Potentials were recorded vs. the reference

electrode Ag/AgCl (sat’d). Reproduced with permission from [29]. Copyright 2007, WILEY-VCH

Verlag GmbH & Co. KGaA, Weinheim


that of complex 1 perhaps because the fluoro substituents make complex 2 more

electrophilic and thus easier to reduce [21]. With the HOMO level being signifi-

cantly shifted and the LUMO level being only slightly changed by fluoro substitu-

tion, the PL emission of complex 2 is significantly blue shifted compared with PL of

complex 1, reflecting a characteristic of Ir complexes that their energy gaps can be

tuned through introducing substituents of different electronic properties on the ppy

ligands.

Tamayo et al. [22] demonstrated tuning of the emission color across a large

portion of the visible part of the spectra by independent tuning of the HOMO

and LUMO levels of the parent complex [Ir(ppy)2(bpy)]PF6, where ppz is 1-phe-

nylpyrazolyl. Highly efficient red (lmax ¼ 635 nm), green (lmax ¼ 542 nm), and

blue (lmax ¼ 492 nm) EL emissions were obtained in LECs based on [Ir

(tbppz)2(biq)]PF6, [Ir(ppy)2(bpy)]PF6, and [Ir(dF-ppz)2(dtb-bpy)](PF6), where tb

is 50-tert-butyl and biq is 2,20-biquinoline [22].Color tuning can also be achieved by destabilization of the LUMO rather than

stabilization of the HOMO. Nazeeruddin et al. [44] achieved blue-green EL emis-

sion (lmax ¼ 520 nm) in LECs based on [Ir(ppy)2(dma-bpy)]PF6, where dma-bpy is

4,40-(dimethylamino)-2,20-bipyridine. Introduction of the electron-donating

dimethylamino groups was demonstrated to significantly destabilize the LUMO

relative to the HOMO when compared with the parent complex [Ir(ppy)2(dtb-bpy)]

PF6, resulting in blue-shifted emission.

Attaching a charged side group on the periphery of the cyclometalating ligands

proposed by Bolink et al. [45] converted a neutral complex into a CTMC. A blue-

green emission (lmax ¼ 487 nm) was observed for [Ir(ppy-PBu3)3](PF6)3. Such a

blue shift in emission, compared with the [Ir(ppy)3] complex, was attributed to

the electron-withdrawing nature of the tri-butylphosphonium group on the phenyl-

pyridine ligand. However, EL was found to change from blue-green to yellow

(lmax ¼ 570 nm) after 100 s of operation at 4 V.

Recently, we have also developed a novel blue-green emitting ppz-based

Ir-complex (stable EL lmax ¼ 488 nm) and a saturated red emitting ppy-based

Ir-complex (PL lmax ¼ 672 nm). Details of photophysical and EL properties of

these two complexes will be described in Sect. 3.

2.3 Lengthening Device Lifetime

The lifetimes of the devices defined as the time it takes for the brightness of the

device to decay from the maximum to half of the maximum under a constant bias

are important figure-of-merit when evaluating the durability of devices for display

or lighting applications. Much effort has been made to lengthen device lifetimes of

LECs.

The rates of electrochemical reactions of emissive CTMCs under electrical

driving are bias dependent. In general, running LEC devices at a higher luminance

120 H.-C. Su et al.

leads to shorter lifetimes, so long lifetime can often be claimed at the expense of

luminance [12, 29, 46–49]. The lifetimes for complex 1 under the 2.6-V driving and

for complex 2 under the 2.9-V driving are ~26 h (extrapolated) and 6.7 h, respec-

tively (Fig. 4). Although operated under a lower current density, the lifetime of the

complex 2 device is significantly shorter than that of the complex 1 device.

Irreversible multiple oxidation and subsequent decomposition under a high electric

field had been proposed as the mechanism for degradation of LECs based on

cationic iridium complexes [32, 50]. Since a higher bias voltage is required for

operating the device based on complex 2 due to its larger energy gap, the higher

electric field in the emissive layer of the complex 2 device perhaps accelerated the

degradation. To mitigate the device degradation, slightly lower bias voltages of

2.5 V and 2.8 V were then applied for testing the devices based on complex 1 and

complex 2, respectively. As shown in Figs. 4a and 4b, slight reduction of the

applied bias by 0.1 V had led to greatly reduced degradation rates for brightnesses

of both devices. The lifetimes of the complex 1 device under the 2.5-V driving and

the complex 2 device under the 2.8-V driving are ~54 and ~12 h (both extrapo-

lated), roughly two times longer than those driven at 0.1-V higher bias voltages.

The longer device lifetimes, however, are achieved at the expense of reduced

brightness. Therefore, emissive materials with high quantum yields are essential

for LECs to be operated at a practical brightness yet using a lowest possible bias,

which would ensure long-lifetime operation of LECs.

Blending CTMCs in an inert polymer can also improve lifetime [14, 16]. It may

result from lowered current density caused by separation of conducting chromo-

phores in LEC devices. The choice of metal contacts was found to influence the

lifetime, even for devices stored in the off state because reactive metal (e.g., Al)

may activate some electrochemical reactions with CTMCs and lead to degradation

[14, 51]. Degradation of [Ru(bpy)3]2þ-based device involves the water-induced

formation of photoluminescence quenchers, which were suggested to be complexes

such as [Ru(bpy)2(H2O)2]2þ and [Ru(bpy)2(H2O)]2O

4þ [36, 52, 53]. Thus, CTMCs

with phenanthroline ligands credited with improved hydrophobicity increased

resistance toward water-induced substitution reactions, leading to the higher

device stability. [Ir(ppy)2(dp-phen)]PF6 device, where dp-phen is 4,7-diphenyl-

1,10-phenanthroline, was found by Bolink et al. to have a lifetime of 65 h, which

is the highest reported for a neat-film Ir-based LEC to date [54].

2.4 Shortening Turn-On Time

Turn-on time is defined as the time required for LEC devices to reach maximum

emission under dc bias. For practical applications, turn-on times must be signifi-

cantly reduced; however, most schemes for improving turn-on time come at a cost

to stability.

Higher applied biases can lead to faster turn-on, but results in shorter lifetimes

[12, 48]. To achieve faster turn-on without losing the stability, a pulsed-biasing


scheme – a short pulse at a higher voltage was used to turn the device on, followed

by a lower voltage that was used to drive the device for extended periods, was

proposed by Handy et al. [12]. However, such technique complicates the driving

circuits and the integration for applications. Reducing the thickness of the emissive

layer of LECs also can reduce the turn-on time, but leads to a decrease in the device

efficiency due to exciton quenching near the electrodes [55]. Smaller counterions

such as BF4� or ClO4

� lead to faster turn-on times than larger one, e.g., PF6�, but

also deteriorate device lifetimes [13, 16]. Increasing ionic conductivity of the

emissive layer by addition of an electrolyte to the CTMC layer has been shown to

improve device lifetimes due to additional ions [20, 32, 56, 57]. However, device

lifetimes were deteriorated as well.

Chemically attaching ionically charged ligands to metal complexes represents

an alternative and promising approach to increase the ionic conductivity and to

improve the turn-on speed of LECs, since the phase compatibility is less an issue in

such configurations. Bolink et al. reported a homoleptic iridium complex containing

ionically charged ligands, giving a net charge of þ3 for each complex [45]. The

LEC based on such complex showed a much improved turn-on time, yet also

suffered other issues, e.g., color shift during operation and low efficiencies, which

are relevant to practical applications as well. Zysmen-Colman et al. recently also

reported homoleptic ruthenium-based and heteroleptic iridium-based CTMCs con-

taining tethered ionic tetraalkylammonium salts [58]. Similarly, LECs constructed

using such complexes also exhibited reduced turn-on times; yet EL efficiencies

from these complexes are not satisfactory, compared with general EL efficiencies

achievable with CTMCs. Hence, further studies on CTMCs containing ionically

charged ligands that can give improved turn-on times, stable operation, and also

high device efficiency are still highly desired.

In one of our previous works, we demonstrated the reduction in turn-on times of

single-component Ir-based LECs with tethered imidazolium moieties. The new

CTMC is achieved by fusing two imidazolium groups at the ends of the two alkyl

side chains of a more conventional complex [Ir(ppy)2(dC6-daf)]PF6 (3)

(where dC6-daf is 9,90-dihexyl-4,5-diazafluorene), forming the new complex [Ir

(ppy)2(dCMIM-daf)](PF6)3 (4) (where dC6MIM-daf is 9,9-bis[6-(3-methylimida-

zolium)hexyl]-1-yl-4,5-diazafluorene) [59]. Molecular structures of complexes 3

and 4 are shown in Fig. 10.

Homogeneous thin films of both complexes 3 and 4 can be obtained by spin-

casting from CH3CN solution. We also tried to prepare a physical blending film of

3 and ionic liquid [BMIM][PF6] for comparative studies; however, the resulting

cloudy films indicated strong phase separation and incompatibility between com-

plex 3 with the long alkyl chains and the ionic liquid with high polar character. The

UV-visible absorption and PL spectra of 3 and 4 in CH2Cl2 solution (10�5 M) and

in neat film (~100 nm) are shown in Figs. 11a and 11b, respectively. Similar

absorption features are observed in either solutions or films for both complexes.

Solution PL emission is in the yellow range (~570 nm) for both complexes 3 and 4,

indicating tethering imidazolium groups at the ends of the alkyl chains in complex 3

has no significant effects on modulating energy gaps.

122 H.-C. Su et al.

The PLQYs of complexes 3 and 4 in CH2Cl2, as determined with a calibrated

integrating sphere, are 0.59 and 0.47, respectively (Table 3). Room-temperature

transient PL of complexes 3 and 4 in CH2Cl2, as determined by the time-correlated

single photon counting technique, exhibits the single-exponential decay behavior,

and the extracted excited-state lifetimes for 3 and 4 are 1.12 ms and 0.94 ms,respectively (Table 3). Spin-coated neat films of complexes 3 and 4 exhibit PL

spectra similar to those observed for their solutions (Fig. 11). The measured PLQYs

and excited-state lifetimes of the neat films are (0.33, 0.66 ms) for 3 and (0.35,

0.75 ms) for 4 (Table 3). Lower PLQYs and shorter excited-state lifetimes in neat

0.0

0.4

0.8

1.2

1.6

2.0a b

0.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

(a.u

.)

Wavelength (nm)

PL, Abs, Solution PL, Abs, Neat filmEL

Inte

nsity

(a.

u.)

200 300 400 500 600 700 800 200 300 400 500 600 700 8000.0

0.4

0.8

1.2

1.6

2.0

0.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

(a.u

)

Wavelength (nm)

PL, Abs, Solution PL, Abs, Neat filmEL

Inte

nsity

(a.

u.)

Fig. 11 Absorption and PL spectra in CH2Cl2 solution (10�5 M) and in neat film along with EL

spectra under a forward bias voltage of 2.8 V for (a) complex 3 and (b) complex 4, respectively.


KGaA, Weinheim

NIr

N

N

NPF6

3

NIr

N

N

N

N

N N

N

3PF6

4

Fig. 10 Molecular structures of complexes 3 and 4


films indicate that interaction between closely packed molecules provides addi-

tional deactivation pathways. However, PLQYs of complexes 3 and 4 in neat films

still retain ~56% and ~75% of those in solutions. Highly retained PLQYs in neat

films of complexes 3 and 4 are likely to be associated with their sterically bulky

dC6-daf and dC6MIM-daf ligands. It is noted that self-quenching in neat films of

complex 4 is not as severe as that in neat films of complex 3 perhaps due to better

steric hindrance provided by imidazolium groups in complex 4.

Figure 12 depicts the electrochemical characteristics of complexes 3 and 4

probed by cyclic voltammetry and the measured redox potentials are summarized

in Table 3. Complexes 3 and 4 exhibit reversible redox peaks at similar potential

1.5 1.0 0.5 0.0 –0.5 –1.0 –1.5–0.015

–0.010

–0.005

0.000

0.005

0.010

0.015

Cur

rent

/mA

E (V vs. Ag / AgCl) / V

3

4

Fig. 12 Cyclic voltammogram of complexes 3 and 4. All potentials were recorded vs. Ag/AgCl

(sat’d) as a reference electrode. Reproduced with permission from [59]. Copyright 2008, WILEY-

VCH Verlag GmbH & Co. KGaA, Weinheim

Table 3 Summary of physical properties of complexes 3 and 4

Complex lmax, PL (nm)a PLQYb t (ms)c Eox1=2

(V)dEred1=2

(V)eDE1/2

(V)fSolutiong Neat

film

Solutiong Neat

film

Solutiong Neat

film

3 568 562 0.59 0.33 1.12 0.66 +1.33 �1.47 2.80

4 570 562 0.47 0.35 0.94 0.75 +1.31 �1.48 2.82aPL peak wavelengthbPhotoluminescence quantum yieldscExcited-state lifetimesdOxidation potentialeReduction potentialfThe electrochemical gap DE1/2 is the difference between Eox

1=2 and Ered1=2

gMeasured in CH2Cl2 (10�5 M) at room temperature


KGaA, Weinheim

124 H.-C. Su et al.

(vs. Ag/AgCl); one reversible oxidation potential occurs at 1.33 V and 1.31 V for

3 and 4, respectively, while one reversible reduction potential occurs at –1.47 V and

–1.48 V for 3 and 4, respectively. The HOMO of [Ir(ppy)2(N^N)]PF6 complexes

has been reported to be a mixture of the iridium and the phenyl group of the C^N

ligand, while the LUMO mostly localizes on the N^N ligand [21]. Since the

structures of complexes 3 and 4 differ only in the terminal alkyl chain of N^N

ligand, their HOMO and LUMO distribution should be similar and give similar

redox potentials. The presence of imidazolium moieties in 4 exhibited limited

influence on the electrochemical properties of the Ir-center, especially the frontier

molecular orbitals.

Device characteristics based on the structure of [glass substrate/ITO/complex

3 or 4 (100 nm)/Ag] are discussed and are summarized in Table 4. EL spectra of the

devices based on complexes 3 and 4 are shown in Figs. 11a and 11b, respectively,

for comparison with their PL spectra. EL spectra are basically similar to PL spectra,

indicating similar emission mechanisms. Time-dependent brightness and current

densities of the LEC devices based on complex 3 operated under 2.7 and 2.8 V are

shown in Fig. 13a. The currents of devices increased slowly with time after the bias

was applied and kept increasing even after 10-h operation. The turn-on time of the

device (tturn-on), defined as the time to achieve brightness of 1 cd cm�2, was 65 min

under 2.8 V. The brightness increased with the current and reached the maximum of

105 cd m�2 at ~500 min under 2.8 V. For devices under 2.7 V, the brightness

reached 56 cd m�2 and was still increasing at 600 min. Comparatively, the LECs

based on complex 4 showed much faster response. As shown in Fig. 13b, this device

turned on sharply in 12 min under 2.8 V, it took only ~200 min for the devices to

reach the maximal brightness of 79 cd m�2 and 31 cd m�2 under 2.8 V and 2.7 V,

respectively. Such improved turn-on times confirm that the chemically tethering

imidazolium groups onto complex 4 increase the density of mobile counterions

(PF6�) in neat films and consequently increase the neat-film ionic conductivity.

Table 4 Summary of the LEC device characteristics based on complexes 3 and 4

Complex Bias (V) lmax, EL (nm)a tturn-on (min)b tmax (min)c Lmax (cd m�2)d �ext, max, �p, max

(%, lm W�1)e

3 2.8 566 65 500 105 5.5, 18.7

2.7 84 >600 >56f 5.5, 19.2

4 2.8 577 12 200 79 4.0, 14.7

2.7 16 200 31 4.5, 17.1aEL peak wavelengthbTime required reaching the brightness of 1 cd cm�2

cTime required to reach the maximal brightnessdMaximal brightness achieved at a constant bias voltageeMaximal external quantum efficiency and maximal power efficiency achieved at a constant bias

voltagefMaximal brightness was not reached during a 10-h measurement


KGaA, Weinheim


Efficient LECs are essential for operation at a practical brightness yet using a

lowest possible bias, which would ensure stable operation of LECs. Hence, it is

important to examine the effect of chemical tethering of imidazolium groups with

an ionic iridium complex on the device efficiencies of LECs. Time-dependent

EQEs and corresponding power efficiencies of the complex 3 and 4 devices under

biases of 2.7 V and 2.8 V are shown in Figs. 14a and 14b, respectively. When a

forward bias was just applied, the EQE was rather low due to unbalanced carrier

injection. During the formation of the p- and n-type regions near electrodes, the

balance of the carrier injection was improved and the EQE of the device thus

increased rapidly. The peak EQE and the peak power efficiency are (5.5%,

18.7 lm W�1) and (5.5%, 19.2 lm W�1) for the complex 3 device under biases of

2.8 V and 2.7 V, respectively. The time-dependent evolution trend in device

efficiency of the complex 4 device is similar to that of the complex 3 device, yet

it took much less time (within 100 min) for the complex 4 device to reach the peak

device efficiency. The peak EQE and the peak power efficiency are (4.0%,

0

20

40

60

80

100

0.0

0.2

0.4

0.6

0.8

1.0

2.7 V2.8 V

Brig

htne

ss (

cd/m

2 )

Time (min)

Cur

rent

Den

sity

(m

A/c

m2 )

0 100 200 300 400 500 600 0 100 200 300 400 500 6000

20

40

60

80

0.0

0.2

0.4

0.6

0.8

2.7 V2.8 V

Brig

htne

ss (

cd/m

2 )

Time (min)

Cur

rent

Den

sity

(m

A/c

m2 )

a b

Fig. 13 Time-dependent brightness and current density under 2.7 V and 2.8 V for the single-

layered LEC device based on (a) complex 3 and (b) complex 4, respectively. Reproduced with

permission from [59]. Copyright 2008, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

0

1

2

3

4

5

6

0

4

8

12

16

20

2.8 V2.7 V

Ext

erna

l Qua

ntum

Effi

cien

cy (

%)

Time (min)

Pow

er E

ffici

ency

(lm

/W)

0 100 200 300 400 500 600 0 100 200 300 400 500 6000

1

2

3

4

5

0

4

8

12

16

20

2.8 V2.7 V

Ext

erna

l Qua

ntum

Effi

cien

cy (

%)

Time (min)

Pow

er E

ffici

ency

(lm

/W)

ba

Fig. 14 Time-dependent EQE and the corresponding power efficiency under 2.7 V and 2.8 V for

the single-layered LEC device based on (a) complex 3 and (b) complex 4, respectively. Repro-

duced with permission from [59]. Copyright 2008, WILEY-VCH Verlag GmbH & Co. KGaA,

Weinheim

126 H.-C. Su et al.

14.7 lm W�1) and (4.5%, 17.1 lm W�1) for the complex 4 device under 2.8 V and

2.7 V, respectively. EL efficiencies of devices based on both complexes 3 and 4 are

comparable, indicating chemical tethering of imidazolium groups with an ionic

iridium complex benefits faster device response yet does not influence EL efficien-

cies significantly. Hence, the technique of integrating ionic liquid moieties onto the

CTMC directly serves as a useful and promising alternative to improve the turn-on

speed of LECs and to realize single-component CTMC LECs compatible with

simple driving schemes.

3 White Light-Emitting Electrochemical Cells Based

on Cationic Transition Metal Complexes

Although solid-state LECs possess several advantages attractive for lighting appli-

cations, rare demonstration of white-emitting solid-state LECs had been reported.

The only previous report of solid-state white LECs was based on phase-separated

mixture of a polyfluorene derivative and polyethylene oxide (PEO) [10]. However,

the fluorescent nature of conjugated polymers would limit the eventual EL effi-

ciency. Recently, CTMCs have also been used in solid-state LECs [11–22, 31, 32,

36, 38, 41, 42, 44–49, 51–60], which show two major advantages over conventional

polymer LECs (a) no ion-conducting material (e.g., PEO) is needed since these

metal complexes are intrinsically ionic; and (b) higher EL efficiencies could be

achieved due to the phosphorescent nature of the transition metal complexes.

Inspired by previous works regarding energy transfer between ionic complexes

reported by Malliaras [38] and De Cola [60], we had previously demonstrated

highly efficient solid-state LEC devices by adopting host–guest cationic metal

complexes [49]. Yet to our knowledge, there is no white LECs based on CTMCs

ever being reported to date, despite their high potential.

Efficient white light emission may bemost easily achieved bymixing two comple-

mentary colors, such as blue-green and red emission. Thus, the development of

efficient blue-green [21, 22, 44, 60] and red-emitting [11–17, 22, 31, 38, 42, 48, 56]

CTMCs is highly desired. In this section, we report the characterization of efficient

blue-green and red-emitting cationic iridium complexes and their successful applica-

tion in white LECs with adopting the effective host–guest strategy [38, 49, 61].

3.1 Photophysical and Electrochemical Properties of NovelBlue-Green and Red-Emitting Cationic Iridium Complexes

Molecular structures of complexes 5–8 are shown in Fig. 15. The emission proper-

ties of complexes 5–8 in solutions (DCM, 10�5 M) or in thin films are summarized

in Table 5. For reducing the response time of LECs, in this work the ionic liquid


[BMIM][PF6] was introduced in the emissive layer to provide additional mobile

anions [29, 32, 49]. Thus, emission properties of 5–7 films in the presence of

[BMIM][PF6] (19 wt%) were also examined. In general, complexes 5–7 show PL

peaks at 491–499 nm and rather high PLQYs of 0.46–0.66 in solutions. High

PLQYs of these complexes indicate that the rigidity of the daf ligand is beneficial

for reducing nonradiative (e.g., vibration) deactivation processes. Among the three

blue-green complexes 5–7, 7 exhibits shortest emission wavelengths of

488–491 nm and highest PLQYs of 0.28–0.30 in thin films (neat or dispersed

with [BMIM][PF6], and thus was chosen as the host material for white LEC studies.

The absorption and PL spectra of blue-green complex 7 and red-emitting complex

8 in solutions and in neat films (~100 nm) are explicitly shown in Fig. 16. Interest-

ingly, complex 7 with the alkyl substitution on daf exhibits nearly same emission

wavelengths in solutions and in solid films. Complex 8 exhibits more saturated red

emission (with PL peaks of 656 and 672 nm in solutions and in neat films,

respectively) compared with that of [Ir(ppyz)2(biq)]PF6 [22]. The energy gaps

estimated by cyclic voltammetry for complex 8 (2.23 eV, Table 5) and [Ir(ppyz)2(biq)]PF6 (2.45 eV) [22] are consistent with the photophysical observation. For [Ir

(ppyz)2(biq)]PF6, the HOMO and the LUMO are predominantly localized on the

ppz and biq ligand, respectively [22]. The smaller energy gap of complex 8, which

also contains biq, is thus associated with destabilization of HOMO in replacing ppz

with ppy.

Cyclic voltammetry was used to probe the electrochemical properties of these

complexes. The results are summarized in Table 5. For complexes 5–7, one

reversible oxidation potential was detected, which can be attributed to the oxidation

that occurred on the Ir center. Interestingly, the electronic nature of ancillary daf

ligands plays a subtle role on the oxidation potential. For example, complex 7

containing a more electro-rich dedaf ligand shows a lower oxidation potential

(1.20 V) when compared with those of complexes 5 (1.29 V) and 6 (1.28 V), both

N NF

F

Ir

NN

F

F

N

N PF6– PF6

–

RR

5: [Ir(dfppz)2 (dasb)]+ (PF6–), R, R = 2, 2′-biphenyl

6: [Ir(dfppz)2 (bmpdaf)]+ (PF6–), R, R = 4-CH3OPh, 4-CH3OPh

7: [Ir(dfppz)2 (dedaf)]+(PF6–), R, R = C2H5, C2H5

N

IrN

N

N

8

+ +

Fig. 15 Molecular structures of complexes 5–8

128 H.-C. Su et al.

containing aryl substitutions on C9 of the diazafluorene ligand. On the contrary, the

reduction capability of the diazafluorene ligand is presumably responsible for

observed reversible reduction of complexes 5–7. Thus, the Ir-complex coordinated

200 300 400 500 600 700 800 9000.0

0.5

1.0

1.5

2.0

2.5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Abs

orba

nce

(a.u

.)

Wavelength (nm)

7, neat film 8, solution 7, neat film 8, solution

PL

Inte

nsity

(a.

u.)

Fig. 16 Absorption (left) and PL spectra (right) of complexes 7 and 8 in dichloromethane

solutions (10�5 M) and in neat films. Reproduced with permission from [61]. Copyright 2008,

American Chemical Society

Table 5 Summary of physical properties of complexes 5–8

Complex lmax, PL (nm), F, t (ms)a Eox1=2

(V)bEred1=2

(V)bDE1/2

(V)Solutionc Neat film or

host–guest film

Film with

[BMIM][PF6]d

5 499, 0.46, 0.71 513, 0.20, 0.36 504, 0.22, 0.42 +1.29e �1.70b 2.99

6 497, 0.66, 0.85 507, 0.22, 0.51 498, 0.26, 0.56 +1.28e �1.72f 3.00

7 491, 0.54, 0.73 491, 0.28, 0.55 488, 0.30, 0.74 +1.20g �1.79f 2.99

8 656, 0.20, 0.75 672, 0.09, 0.33 – +0.86f �1.37f 2.23

8 (0.2 wt

%) : 7

– (493, 606), 0.26,

(0.49h, 1.54i)

(488, 603), 0.29,

(0.54h, 1.68i)

– – –

8 (0.4 wt

%) : 7

– (493, 614), 0.27,

(0.38h, 1.52i)

(488, 612), 0.28,

(0.47h, 1.68i)

– – –

aAt room temperaturebPotential vs. ferrocene/ferrocenium redox couplecMeasured in CH2Cl2 (10

�5 M)dFilms containing 19 wt% [BMIM][PF6]e0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6) in acetonitrilef0.1 M TBAP6 in acetonitrileg0.1 M TBAPF6 in CH2Cl2hMeasured at 480 nmiMeasured at 650 nm

Reproduced with permission from [61]. Copyright 2008, American Chemical Society


to an electron-rich dedaf ligand (complex 7) shows a more negative reduction

potential (�1.79 V) when compared with those complexed to the aryl-substituted

daf ligand [complexes 5 (�1.70 V) and 6 (�1.72 V)]. These results thus suggest

that the energies of frontier orbitals in these complexes can be subtly manipulated

by tailoring the electronic properties of the auxiliary daf ligands. For the red

complex 8, one reversible oxidation (0.86 V) and one reversible reduction

(�1.37 V) were detected. The electron-rich character of the ppy ligand and the

more p-delocalized biq ligand contribute to the higher ease of oxidation and

reduction, respectively, thus leading to a small energy gap of complex 8.

Emission properties of the 7:8 host–guest films are also summarized in Table 5.

Figure 17 depicts the PL spectra of the host–guest films with different guest

8 concentrations (0.4 and 0.2 wt%) with or without the presence of [BMIM][PF6]

(19 wt%). In general, with the presence of [BMIM][PF6], both the host and guest

exhibit longer excited-state lifetimes and the PLQYs are slightly raised (Table 5),

indicating the role of [BMIM][PF6] in suppressing intermolecular interactions [49].

As shown in Fig. 17, white emission of different blue-green and red compositions

can be achieved by adjusting the guest concentration. Raising the guest concentra-

tion effectively enhances the host–guest energy transfer, accompanied by shorter

lifetimes of the host emission (Table 5). The host–guest film containing 0.4 wt% of

8 (guest) shows white emission having CIE coordinates of (x, y) ¼ (0.34, 0.37),

which is rather close to the ideal equal-energy white (x, y) ¼ (0.33, 0.33), and

therefore was subjected to further EL studies.

400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

1.2Without BMIM+PF6

–

8 (0.4 wt.%) 8 (0.2 wt.%)

With BMIM+PF6–

8 (0.4 wt.%) 8 (0.2 wt.%)

PL

Inte

nsity

(a.

u.)

Wavelength (nm)

Fig. 17 PL spectra of host–guest films containing different guest concentrations (0.4 and 0.2 wt

%) without and with [BMIM][PF6] (19 wt%). Reproduced with permission from [61]. Copyright

2008, American Chemical Society

130 H.-C. Su et al.

3.2 Electroluminescent Properties of White Light-EmittingElectrochemical Cells Based on Host–Guest CationicIridium Complexes

The LECs have the structure of ITO/emissive layer (100 nm)/Ag (150 nm), where

the emissive layer contains host 7 (80.5 wt%), guest 8 (0.4 wt%), and [BMIM][PF6]

(19.1 wt%). Their EL properties are summarized in Table 6. The EL spectra of the

white LECs under various biases, along with the PL spectra for comparison, are

shown in Fig. 18a. It is noted that the peak wavelength of the blue component in EL

spectra is 488 nm, which is one of the shortest EL wavelengths reported to date for

LECs based on CTMCs. Compared with PL, the relative intensity of the red emission

with respect to the blue emission is larger in EL and increases as the bias decreases.

Furthermore, the maximum current density of the host–guest device (with 0.4 wt%

Table 6 Summary of white LEC device characteristics

Bias (V) CIE (x, y)a CRIa tmaxb (min) Lmax

c

(cd m�2)

�ext, max, �L, max, �p, maxd

(%, cd A�1, lm W�1)

t1/2e (h)

2.9 (0.45, 0.40) 81 240 2.5 4.0, 7.2, 7.8 8.9

3.1 (0.37, 0.39) 80 60 18 3.4, 6.1, 6.2 1.3

3.3 (0.35, 0.39) 80 30 43 3.3, 5.8, 5.5 0.4aEvaluated from the EL spectrabTime required to reach the maximal brightnesscMaximal brightness achieved at a constant bias voltagedMaximal external quantum efficiency, current and power efficiencies achieved at a constant bias

voltageeThe time for the brightness of the device to decay from the maximum to half of the maximum

under a constant bias voltage

Reproduced with permission from [61]. Copyright 2008, American Chemical Society

0.0

0.4

0.8

1.2

1.6PLEL 3.3 VEL 3.1 VEL 2.9 V

Inte

nsity

(a.

u.)

Wavelength (nm)400 500 600 700 800 2.9 3.0 3.1 3.2 3.3

0

1

2

3

4

5

6Guest 0.4 wt.%Guest 0 wt.%

Guest

–5.66 eV

–3.43 eV

Host

–6.00 eV

–3.01 eV

Max

imum

Cur

rent

Den

sity

(mA

/cm

2)

Voltage (V)

ba

Fig. 18 (a) The bias-dependent EL spectra of the white LECs compared with the PL spectrum.

(b) Maximum current density vs. voltage characteristics for the host–guest (0.4 wt% doping

concentration) and the host-only LECs. Inset: the energy level diagram of the host and guest

molecules. Reproduced with permission from [61]. Copyright 2008, American Chemical Society


guest) is lower than that of the host-only device under same bias conditions

(Fig. 18b). These results could be understood by energy level alignments of the

host and guest (inset of Fig. 18b). At lower biases, such energy level alignments favor

carrier injection and trapping on the smaller-gap guest, resulting in direct carrier

recombination/exciton formation on the guest (rather than host– guest energy trans-

fer). Therefore, larger fractions of guest emission are observed at lower biases.

Nevertheless, white emission with CIE coordinates of (0.35, 0.39) could be achieved

at higher biases and brightness. Also, the present white LECs exhibit a rather high

color rendering index (CRI) (up to 80), which is an important characteristic for solid-

state lighting.

Figure 19a shows the time-dependent brightness and current density under

constant biases of 2.9–3.3 V for the white LEC. After the bias was applied, the

current first rose and then stayed rather constant. On the contrary, the brightness

first increased with the current and reached the maxima of 2.5 cd m�2, 18 cd m�2,

and 43 cd m�2 under biases of 2.9 V, 3.1 V, and 3.3 V, respectively. The brightness

then dropped with time with a rate depending on the bias voltage (or current).

Corresponding time-dependent EQEs and power efficiencies of the same device are

shown in Fig. 19b. When a forward bias was just applied, the EQE was rather low

due to poor carrier injection. During the formation of the doped regions near

electrodes, the capability of carrier injection was improved and the EQE thus rose

rapidly. The peak EQEs, current and power efficiencies at 2.9 V, 3.1 V, and 3.3 V

are (4.0%, 7.2 cd A�1, 7.8 lm W�1), (3.4%, 6.1 cd A�1, 6.2 lm W�1), and (3.3%,

5.8 cd A�1, 5.5 lm W�1), respectively.

Peak brightness and turn-on time (the time required to reach the maximal

brightness) as a function of bias voltage for white LECs are shown in Fig. 20a.

An electrochemical junction between p- and n-type doped layers of LECs is formed

during device operation. As revealed in previous studies [31], as bias voltage

increases, the junction width decreases due to extension of doped layers, conse-

quently leading to higher current density (Fig. 19a) and higher brightness (Fig. 20a).

The higher electric field in the device also accelerates redistribution of mobile ions,

0

10

20

30

40

50

0.01

0.1

1

10

2.9 V3.1 V3.3 V

Brig

htne

ss (

cd/m

2 )

Time (min)

Cur

rent

Den

sity

(m

A/c

m2 )

0 100 200 300 400 500 600 0 100 200 300 400 500 6000

1

2

3

4

5

0

2

4

6

8

2.9 V 3.1 V 3.3 V

Ext

erna

lQ

uant

um E

ffici

ency

(%

)

Time (min)

Pow

er E

ffici

ency

(lm

/W)

ba

Fig. 19 (a) Brightness (solid symbols) and current density (open symbols) and (b) external

quantum efficiency (solid symbols) and power efficiency (open symbols) as a function of time

under a constant bias voltage of 2.9–3.3 V for the white LEC. Reproduced with permission from

[61]. Copyright 2008, American Chemical Society

132 H.-C. Su et al.

which facilitates formation of ohmic contacts with the electrodes. Thus, operation

of LECs under higher bias voltages speeds up the device response (Fig. 20a).

However, higher brightness and faster response are obtained at the expense of

device stability. As shown in Fig. 20b, peak EQE and device lifetime (the time

for the brightness of the device decaying from the maximum to half of the

maximum under a constant bias voltage) of the white LECs deteriorate under higher

bias voltages. It may be associated with the fact that the higher electric field or

current density accelerates degradation (multiple oxidation and subsequent decom-

position) [32] of the emissive CTMCs. Previous studies showed that addition of

materials altering the electronic properties of the emissive layer of LECs also

influence the device lifetimes [14, 56]. Blending an inert polymer [14] in the

emissive layer of LECs improves device lifetimes, while addition of an electrolyte

[56] shortens device lifetimes. Blending of an inert polymer reduces the electronic

conductivity of the emissive layer and thus lowers the current density. On the

contrary, additional mobile ions provided by the electrolyte increase the doping

level of the doped region, resulting in higher current density due to increased

electronic conductivity. These results also imply that higher current leads to shorter

device lifetimes. Recently, two reports [52, 53] have revealed that electrical driving

of LECs based on CTMCs induces oxo-bridged dimers, which effectively quench

EL. However, detailed degradation mechanisms of the LECs based on CTMCs

remain unclear and further studies are still needed to achieve practical device

lifetimes.

4 Outlook

Solid-state CTMC-based LECs exhibit advantages of simple fabrication processes,

low-voltage operation, and high power efficiency, which thus may be competitive

for lighting applications. However, improvements will be required in our prototype

0

10

20

30

40

50

0

40

80

120

160

200

240

Brig

htne

ss (

cd/m

2 )

Voltage (V)

Brightness

Tur

n-on

Tim

e (m

in)

Turn-on Time

2.9 3.0 3.1 3.2 3.3 2.9 3.0 3.1 3.2 3.33.2

3.4

3.6

3.8

4.0

4.2

0

2

4

6

8

10

Ext

erna

lQ

uant

um E

ffici

ency

(%

)

Voltage (V)

ExternalQuantum Efficiency

Life

time

(hr)Lifetime

ba

Fig. 20 (a) Peak brightness (solid symbols) and turn-on time (open symbols) and (b) peak externalquantum efficiency (solid symbols) and lifetime (open symbols) as a function of bias voltage for thewhite LECs. Reproduced with permission from [61], American Chemical Society


white CTMC-based LECs for practical applications. For device efficiencies,

measured device efficiency did not reach the upper limit (~6%) estimated from

the PLQY of the emissive layer (~30%) and ~20% light out-coupling efficiency

from a typical layered light-emitting device structure. It may be associated with

exciton quenching by one of the electrodes if there is discrepancy in electron and

hole transport, rendering the recombination zone closer to one of the electrodes. To

further improve the device efficiency, further tailoring of molecular structures to

achieve more balanced electron and hole transport is required. Improving device

lifetimes will also be essential for lighting applications. Introducing functional

groups with resistance against quencher formation and/or electrochemical degrada-

tion in CTMCs will facilitate realization of long-lifetime white LECs for lighting

applications.

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